The invention concerns a photoelectric distance measuring device, with a source of light, a reflecting scale grating, a reference grating which is transparent and displaceable with respect to the scale grating and which has a different lattice constant, an air space between the two gratings and several photoelectric receivers placed at intervals in the direction of the distance to be measured, said receivers scanning the scale grating via the reference grating.
A distance measuring system in which the scale and reference gratings are aligned with their spacings parallel to each other is known from U.S. Pat. No. 2,886,717. The two gratings display slightly differing lattice constants, and a slight air space is located between the two gratings. Two diaphragm openings are arranged in the path of the illuminating beam emitted by a source of light; two different fields of the grating system are illuminated through the openings.
Distance measurement takes place perpendicularly to the direction of the divisions of both gratings. This creates a strip pattern with strips located parallel to the direction of the spacings of the gratings; the strips travel perpendicularly to the direction of the spacings if the gratings move relatively with respect to each other. The diaphragm openings are located adjacent to each other in the measuring direction and are at a distance from each other so that the image sections of the strip patterns illuminated by them are out of phase with respect to each other by a fraction of one-half of the strip period. Associated with each image section are photoelectric receivers which convert the modulation of the light caused by the movement of the strips into electrical displaced-phase measuring signals.
Because of the special arrangement of the diaphragm openings, it is possible to determine not only the displacement distance of the scale grating, but also the direction of the displacement. Because the measuring signals are derived from light fluxes which had penetrated different partial ranges of the grating system, local differences in the optical properties of these ranges affect the accuracy of the measurement. Specifically, these consist of locally different degrees of soiling and errors in the scale spacings.
The known device uses amplitude gratings as the reference and scale gratings. It is immediately obvious that with a web/gap ratio of 1:1, the optically opaque web of the reference grating even initially suppresses 50% of the illuminating light. To the extent that the web does not entirely absorb this portion of the light, additional interference reflections are created on the part of the receivers. The same percentage loss of light occurs upon the emittance of the light reflected by the scale grating from the measuring system.